Homing missiles include a seeker of some kind, to indicate the direction of a target. The seeker will include a sensor, which will be sensitive to radiation emitted by, or reflected from, the target. Different kinds of radiation can provide different information about a target. For example, radar can give very accurate information about range to a target, but to obtain angular information from radar reflections requires more complex equipment and processing. Also, sensors for detecting different kinds of radiation perform differently in different environmental conditions and over different ranges; for example, infrared (IR) radiation can provide images as well as positional and directional information, but has a shorter range than radar and can be adversely affected by poor weather conditions. It is known to provide multiband sensors, which take advantage of the complementary nature of different kinds of radiation by detecting RF radiation and also electromagnetic radiation of at least one other waveband, for example near IR.
A well-known arrangement for a sensor is the Cassegrain telescope. A Cassegrain telescope comprises two focusing mirrors having a common centre of curvature. One of the mirrors—the primary mirror—is concave, with its focus at the common centre of curvature. The primary mirror has a transparent region or a hole its centre. The other mirror—the secondary mirror—is arranged between the primary mirror and the common centre of curvature and is convex, facing towards the primary mirror and away from the common centre of curvature. The secondary mirror has a virtual focus at the common centre of curvature (i.e. parallel rays striking the secondary mirror are reflected as divergent rays appearing to originate at the common centre of curvature). Radiation striking the primary mirror is focused towards the secondary mirror, which in turn focuses the radiation through the transparent region or hole in the primary mirror, towards a radiation detector arranged behind the primary mirror.
Several multiband sensors for missile seekers have been proposed in the prior art. For example, U.S. Pat. No. 2,972,743 (Svensson et al.) describes a multiband sensor in which a Cassegrain telescope is provided for the detection of infrared radiation, but which also includes an RF sensing subsystem in the form of an RF reflector, which focuses incoming RF radiation onto a RF detector. The RF reflector is arranged between the primary and secondary mirrors of the Cassegrain telescope, but transmits IR radiation as it is in the form of a wire mesh. The RF reflector is mounted with and coaxial to the secondary reflector.
U.S. Pat. No. 3,165,749 (Cushner) describes a multiband sensor in which a Cassegrain telescope is again provided for the detection of infrared radiation. An IR imager is provided. In this arrangement, the primary mirror reflects RF radiation as well as IR radiation. The secondary mirror is reflective to IR but transmissive to RF, and an RF horn is positioned behind the secondary mirror. A similar arrangement is described in U.S. Pat. No. 4,866,454 (Droessler et al.). US 2010/0127113A1 (Taylor et al.) describes another similar system also including baffles to block unwanted sunlight from reaching the IR detector. US 2012/0080552A1 (Taylor et al.) describes another similar system in which the secondary mirror is a molded mirror.
U.S. Pat. No. 7,183,966 (Schramek et al.) describes examples of multiband sensors that detect microwave radiation and light-wave radiation a first frequency and a second frequency. The sensors described include a Cassegrain telescope for the light-wave radiation. The primary mirror of the Cassegrain telescope is transparent to the microwave radiation. A system is described that includes, in addition to an RF detector, detectors for detecting pulses of radiation generated by a semi-active laser (SAL) system and for detecting images formed by radiation generated by a semi-active laser system and images formed by IR radiation. Three paths are provided for the IR radiation: Cassegrain telescope arrangements for SAL imaging and IR imaging, and a form of folded-Cassegrain telescope arrangement for SAL pulse detection. In the SAL pulse detector, the secondary mirror directs the SAL pulses to a plane mirror, which directs them back through an aperture at the centre of the secondary mirror to an avalanche photodiode or other detector behind the secondary mirror. The RF radiation is essentially independent of the IR Cassegrain telescopes.
The amount of space in a missile is limited. It is desirable to include further detectors or other apparatus in the missile, whilst keeping the space taken up by the detectors small.
It would be advantageous to provide a sensor for a missile seeker in which one or more of the aforementioned disadvantages is eliminated or at least reduced.